Human VLDLs assembled in the liver and secreted into the circulation supply energy to peripheral tissues. VLDL lipolysis yields atherogenic LDLs and VLDL remnants that strongly correlate with CVD. Although the composition of VLDL particles has been well-characterized, their 3D structure is elusive because of their variations in size, heterogeneity in composition, structural flexibility, and mobility in solution. Here, we employed cryo-electron microscopy and individual-particle electron tomography to study the 3D structure of individual VLDL particles (without averaging) at both below and above their lipid phase transition temperatures. The 3D reconstructions of VLDL and VLDL bound to antibodies revealed an unexpected polyhedral shape, in contrast to the generally accepted model of a spherical emulsion-like particle. The smaller curvature of surface lipids compared with HDL may also reduce surface hydrophobicity, resulting in lower binding affinity to the hydrophobic distal end of the N-terminal &beta;-barrel domain of cholesteryl ester transfer protein (CETP) compared with HDL. The directional binding of CETP to HDL and VLDL may explain the function of CETP in transferring TGs and cholesteryl esters between these particles. This first visualization of the 3D structure of VLDL could improve our understanding of the role of VLDL in atherogenesis.

f1: Electron micrographs of VLDL particles by negative-staining EM and cryo-EM. A: Survey view of the negative-staining VLDL particles. B: Representative negative-staining particles sorted by size. C: A field of view of VLDLs kept at 4°C before plunge-freezing. D: Representative particles at 4°C sorted by size. Background variation is because of differences in the thickness of the ice where the particles are embedded. E: Survey view of VLDLs kept at 40°C before plunge-freezing. F: Representative particles at 40°C sorted by size. Arrowheads indicate the vertices on the VLDL particles. G: In total, ∼600 VLDL particles are divided into 30, 40, 50, and 60 nm size groups (n is the number of particles in each group). The average smallest angles of these groups are 67, 60, 55, and 45°, respectively. The error bars represent the standard deviations. The inset indicates how the particle circumference is marked, and the smallest angle θ is taken. Scale bars: 50 nm (A, C, E); 20 nm (B, D, F).

f1: Electron micrographs of VLDL particles by negative-staining EM and cryo-EM. A: Survey view of the negative-staining VLDL particles. B: Representative negative-staining particles sorted by size. C: A field of view of VLDLs kept at 4°C before plunge-freezing. D: Representative particles at 4°C sorted by size. Background variation is because of differences in the thickness of the ice where the particles are embedded. E: Survey view of VLDLs kept at 40°C before plunge-freezing. F: Representative particles at 40°C sorted by size. Arrowheads indicate the vertices on the VLDL particles. G: In total, ∼600 VLDL particles are divided into 30, 40, 50, and 60 nm size groups (n is the number of particles in each group). The average smallest angles of these groups are 67, 60, 55, and 45°, respectively. The error bars represent the standard deviations. The inset indicates how the particle circumference is marked, and the smallest angle θ is taken. Scale bars: 50 nm (A, C, E); 20 nm (B, D, F).

Human VLDLs assembled in the liver and secreted into the circulation supply energy to peripheral tissues. VLDL lipolysis yields atherogenic LDLs and VLDL remnants that strongly correlate with CVD. Although the composition of VLDL particles has been well-characterized, their 3D structure is elusive because of their variations in size, heterogeneity in composition, structural flexibility, and mobility in solution. Here, we employed cryo-electron microscopy and individual-particle electron tomography to study the 3D structure of individual VLDL particles (without averaging) at both below and above their lipid phase transition temperatures. The 3D reconstructions of VLDL and VLDL bound to antibodies revealed an unexpected polyhedral shape, in contrast to the generally accepted model of a spherical emulsion-like particle. The smaller curvature of surface lipids compared with HDL may also reduce surface hydrophobicity, resulting in lower binding affinity to the hydrophobic distal end of the N-terminal &beta;-barrel domain of cholesteryl ester transfer protein (CETP) compared with HDL. The directional binding of CETP to HDL and VLDL may explain the function of CETP in transferring TGs and cholesteryl esters between these particles. This first visualization of the 3D structure of VLDL could improve our understanding of the role of VLDL in atherogenesis.